1. Field of the Invention
It is often necessary to switchably insert one or more quadripoles in microwave transmission lines for modifying the transmission thereof.
One particularly advantageous application of the invention concerns the construction of a variable attenuator comprising several cells with graduated values. In other applications, amplifiers, filters or frequency changers will have to be switchably inserted in the line.
2. Description of the Prior Art
This problem is usually solved by means of switching members called "coaxial switches", although the line element which forms them is not necessarily coaxial. These switches generally have the disadvantage of using two mobile line segments which, depending on their position, provide direct transmission or transmission through the quadripole to be inserted.
They introduce a high insertion loss and have relatively poor isolation between the two transmission channels. In general, they are expensive and have a substantial electric energy consumption.
U.S. Pat. No. 3,319,194 describes a variable attenuator whose switching member is of the above mentioned type, with the particularity that it is formed by a micro strip line whose central conductor forms, at intervals, two parallel channels one of which provides direct transmission and the other transmission with attenuation, switching between the two channels being achieved by means of a flexible conducting segment movable from one channel to the other.
This construction provides good impedance continuity and a low insertion loss but has the disadvantage of forming a parasite passage channel at the terminals of each attenuation cell when this latter is switched.
Another disadvantage is that, since the contacts are switched for a relatively long time during switching, considerable intermediate attenuation is then obtained, which is undesirable for some applications.
3. Object of the Invention
It is an object of the present invention to provide a switchable transmission line element constructed according to the micro strip line technique, but free of the above mentioned disadvantages and particularly economical.